It can be desirable for polyethylene copolymers to possess a broad orthogonal composition distribution (“BOCD”), as this may enhance the stiffness, toughness, and processability (S/T/P) balance of the polyethylene and compositions that include these polymers. Given that polymers are a blend of molecules having a distribution of different chain-lengths, polymers having BOCD are branched polymers that have a preponderance, if not all, of any branching that may occur on the high molecular weight molecules of the polymer, making them less crystalline. This microstructure has a tendency to improve certain properties of products made from such BOCD-type polymers. For example, in film grades of linear low density polyethylene (“LLDPE”) with the BOCD microstructure, improved processability (e.g., higher I21/I2) is achieved without sacrificing toughness (e.g., dart impact) and stiffness (e.g., tensile moduli) in comparison to LLDPE's that lack BOCD structure. Analogously, properties such as Slow Crack Growth Resistance are improved for pressure-pipe and blow-molding grades (esp. high density polyethylene, “HDPE”) that have a bimodal molecular-weight distribution with a “reversed comonomer distribution” structure, or BOCD-type structure.
Ziegler-Natta (“ZN”) produced polyethylenes tend not to have a BOCD-type structure, most of the short-chain branching being on the low molecular weight portion of the molecules thus produced. Metallocene polyethylenes, on the other hand, often do have BOCD-type structure. The value of BOCD polymers relative to conventional polymers is significant. Accordingly, any methodology enabling quantification of BOCD character in a non-trivial manner will certainly have a dramatic impact on the ability to develop new products, gain structure-property insights, and guide catalyst discovery.
A challenge in developing such methodology has been in designing appropriate characterization methods and metrics that are sensitive to the level of such microstructure attributes (i.e., “BOCD-ness” vs “ZN-ness”, etc.). The Cross-Fractionation Chromatography (“CFC”) technique discussed in WO 2015/123164 A1 provides a measure of the bivariate mass distribution (“BVMD”) of the crystallized portion of polyethylene copolymers, and it is well-suited to elucidate the BOCD nature of such resins. Often, a visual examination of the BVMD contour plot is sufficient to confirm that the distribution is BOCD-like as opposed to the conventional ZN-type, such as is shown in FIG. 1. Such an evaluation consists of a non-quantitative method of ordering the polyethylene copolymers under consideration by increasing or decreasing BOCD-like character based on the “tilt” of the BVMD contours in the 2D plane; for example, in FIG. 1 the slope of the line in (a) is less than 0 (negative “tilt”) whereas the slope of the line in (b) is greater than 0 (positive “tilt”), and this allows one to conclude that (a) has more BOCD-like character than (b).
WO 2015/123164 A1 discloses a method called “Equal-Halves Analysis” (“EHA”) to obtain the weight-average temperatures (Tw1 and Tw2) and weight-average molecular weights (Mw1 and Mw2) of the polymer divided in half by mass; the differences and ratios of these metrics in conjunction with the MIR are able to differentiate resins with superior S/T/P attributes from others. However, the method has certain drawbacks, including (1) the EHA method is conceived for distributions that are separated along a temperature axis and hence these metrics are not universally applicable; and (2) the method excludes fractions below a cutoff in weight percent for which the signal-to-noise ratio is too low for Gel Permeation Chromatography (“GPC”) data processing.
It would be desirable to utilize the entire BVMD with contributions from all molecular weight fractions even if their molecular-weight averages are not typically reportable using GPC alone. What is needed is an improved method of determining the BOCD attribute of polymers, especially polyethylenes.
Relevant documents also include U.S. Pat. No. 7,985,593, U.S. Pat. No. 9,095,792, and H. T. Liu et al. “Bimodal Polyethylene Products from UNIPOL™ Single Gas Phase Reactor Using Engineered Catalyst”, in 195 MACROMOLECULAR SYMPOSIA 309-316 (2003).